One of the most widely used curing agents for liquid epoxy resins is a liquid anhydride. There are a number of such anhydrides available on the market for this purpose; including methyltetrahydrophthalic anhydride (MTHPA), maleic anhydride adduct of methylcyclopentadiene (nadic methyl anhydride) and dodecenylsuccinic anhydride, as well as liquid eutectic mixtures of MTHPA and other anhydrides. In almost every case a promoter and an external source of heat are required to achieve the requisite degree of cure.
Unfortunately, the initiation temperature for the liquid epoxyanhydride reaction is well above ambient for most of the currently available liquid anhydrides. In addition, the viscosity index of liquid epoxy resins shows a sharp drop from about 12,000 cps at ambient (25.degree. C.) to about 100 cps at 60.degree. C., the temperature at which the liquid epoxy-anhydride reaction is initiated by only the most reactive systems.
The commercially available liquid anhydrides currently on the market show very little change in viscosity upon being heated, going only from about 100 cps at ambient temperature to about 20 cps at 60.degree. C. The net effect, therefore when such an epoxy-anhydride system is rapidly heated to 60.degree. C. is an abrupt loss of viscosity which continues to decrease to a low of about 40 cps as the system is heated from 60.degree. C. to 90.degree. C. to effect gelation.
Filament winding of glass, carbon, Kevlar (aromatic polyimide) or other fibrous materials with an epoxy-anhydride system to produce tubing, piping and other geometric structures constitutes one of the largest end use areas for liquid anhydrides. In this work the fibers are drawn through a bath of the epoxy resin-anhydride mixture and then spirally wound upon a rotating mandrel. The impregnating bath travels along the length of the mandrel and layer after layer of impregnating material is applied to the fiber until the required thickness is obtained. Since the thickness may vary from 10 mils to several inches, the time in which the impregnating system is held at impregnating temperature (usually 40.degree. to 60.degree. C.) can vary from 10 minutes to 3 or more hours. After impregnation, the wound tube is either heated internally or moved into an oven to achieve cure. In either case there is sizeable drain-off of resin during the winding and upon the application of heat, before gelation takes place. As a result, wound structures having low resin content and containing many internal voids are produced, many of which delaminate after high temperature cure. Additionally, although the liquid drain-off is returned to the impregnating tank for re-use, the anhydride in this portion has been so exposed to air and humidity that it is converted to the free acid. Polymers made with this drain-off material are less highly crosslinked, since an acid can react only once with an epoxy resin, and the hydroxyl group formed and left unreacted tends to make the polymer water sensitive.
The viscosity needs to be controlled. The ideal system for the impregnation of fibers can be defined as one that is low enough in viscosity to allow release of entrapped air, and yet high enough in viscosity so that liquid material does not run out of the impregnated structure before gelation takes place. Experience has shown that viscosities above about 250 cps are required to prevent run-off, and that above the 1000 cps level, air release becomes a problem.
The viscosity of the epoxy-anhydride system is likewise important in other uses such as for casting, molding and the like. In most epoxy encapsulating systems, for example, whether they are to be cast or molded around electrical components such as coils, transformers, or other devides, large amounts of finely divided (-325 mesh) particulate mineral fillers are added. This is done to impart thermal shock resistance by reducing the coefficient of expansion of the system, and to moderate the peak exotherm that occurs in thick sections, and to impart flame retardence. If the viscosity of the system drops upon heating and curing, the problem of filler settling is encountered which tends to make the cured casting more sensitive to thermal shock, with less flame retardence and inferior electrical properties in the areas of low filler content.